1. Field of the Invention
The present invention relates to a process and equipment for micro-pattern forming on the surface of a rolling roll in forming a precise pattern having a number of projections and indentations in the same surface, metal sheets obtained through the rolling roll which are to be pressed and painted like outer panels of automobiles for example, and a method of preparing the metal sheets.
2. Prior Art
The beautiful appearance of a car body has recently come to be keenly demanded. Actually, a car body indistinctly reflecting a surrounding scene in the surface thereof is becoming no longer appealing to car owners; that is, the car owners demand a car that sharply reflects a surrounding scene in the body surface. The sharpness of reflection of the scene thus reflected in the body surface is evaluated by a DOI value by measuring the rate of reflection of a slit light that has entered the surface of the car body at a given angle. The larger the value thus obtained, the higher the sharpness of reflection in the car body surface.
Nowadays, there is adopted a method of forming a number of projections and indentations in the surface of a rolling roll by projecting pulses of a laser beam directly to the surface of the roll, and transferring a pattern of the projections and indentations from the roll surface to the surface of a cold-rolled steel sheet by the use of a roll having projections and indentations formed therein, in a skin pass rolling process after the annealing of the steel sheet.
A dull-surface skin pass rolling roll with the surface roughened by forming projections and indentations by shot blasting has been popularly used in skin pass rolling, thereby transferring a pattern of projections and indentations to the surface of the steel sheet, and steel sheets thus obtained are used as a material for pressworking.
The pattern with projections and indentations formed in the roll surface by the use of a laser beam differs from that formed through a conventional shot blasting process or an electrical-discharge dulling process. The projections and indentations are uniform in size and formed at a fixed pitch throughout the roll surface. Therefore, steel sheets processed by this process and painted have the advantage that the surface thereof has better sharpness of reflection, and has excellent resistance to die galling at the time of pressing.
Prior art disclosed in Japanese Patent LaidOpen Nos. 55-94790 and 56-119687 and Japanese Patent Publication No. 58-25557 are the processes described above or variations thereof.
Conventional laser beam machining technology has the following problems.
(1) Since the laser bean is directly applied to the roll surface to make indentations, a laser-beam oscillator is required to put out a high power of 1 kW or more for efficient machining of a work, such as a rolling roll having a wide surface area. It is, therefore, natural to adopt a CO.sub.2 laser. This CO.sub.2 laser, however, is of a large size and needs larger maintenance costs and labour.
(2) In the pattern of projections and indentations thus produced by such a laser-beam machining equipment, each indentation is likely to become a ring-like or of similar form with molten metal deposited around it. It is therefore impossible to form a pattern of projections and indentations into a free shape.
(3) The pattern of projections and indentations formed by melting the metal with a laser beam is composed of austenite in the projection-indentation section. A roll having projections and indentations thus formed has low wear resistance when used in rolling.
(4) The diameter of each indentation of the pattern of projection and indentation is determined by the diameter of the laser beam converged by a condenser. This diameter can not physically be decreased below about 100 .mu.m because of the long (10.6 .mu.m) wavelength of the CO.sub.2 laser beam.
(5) To insure efficient machining of a wide surface area, it is necessary to produce a laser beam of very high-frequency pulse wave. In the case of the CO.sub.2 gas laser, however, this can not be realized by electrical means such as Q switching. Accordingly, a mechanical chopper has been used for this purpose. This method, however, also has a problem that high mechanical chopper and roll speeds and phases do not necessarily match each other and the pattern obtained has projections and indentations irregularly arranged.
As a means to solve the above-mentioned problems, a photo-etching method used in pattern forming on roll surface in a gravure printing machine is adopted.
This photo-etching process is adopted to produce a rolling roll having a roughened surface with a uniform projection and indentation pattern. Steel plates manufactured by use of the roll have excellent pressworking and decorative properties as long known, for example as disclosed in Japanese Patent Publication Nos. 41-14973 and 46-19535. However, this process has not yet been put into fully practical use because of its low roll processing efficiency and very high cost.
The photo-etching process, as well known, may be broken down into the following steps.
(1) Activation of roll surface PA1 (2) Application of resist (photosensitive corrosion-resistant agent) PA1 (3) Drying PA1 (4) Film affixing PA1 (5) Exposure to light PA1 (6) Removal of film PA1 (7) Development PA1 (8) Drying PA1 (9) Etching PA1 (10) Removal of resist PA1 (11) Post-treatment (washing, neutralization, etc.) PA1 (1) Forming a film of acid corrosion resistant resin on the rolling roll surface by applying a mixture of an acid corrosion-resistant resin solution and a light-absorbing agent; PA1 (2) Marking off a desired pattern continuously on this film of resin, using a Q switch and a YAG laser having an output of 5 to 100 W on the average, and exposing the roll surface with the pattern thus marked off; and PA1 (3) Thereafter, etching to form the desired pattern on the roll surface. PA1 (a) a roll rotating device for supporting a roll of workpiece and rotating it; PA1 (b) a coating device which is similar to a spray device or a similar device installed at a side of the above-mentioned roll rotating device so as to apply resin substance to the surface of a roll while the resin is in its liquid state; PA1 (c) a laser plotter comprising, in combination, an oscillator having an output of 5 to 100 W and generating a pulse-like laser in response to an electrical signal and a series of devices including a lens for use in guiding the laser to the surface of the roll and focusing it on a mirror; PA1 (d) an etching device comprised of a spray or the like for use in blowing etching liquid to the surface of the roll processed with the abovementioned laser plotter PA1 (e) a moving device for use in moving in an axial direction of a roll the above-mentioned coating device, laser plotter and etching device independently or simultaneously; PA1 (f) an image processing device for making a desired pattern to be formed on the surface of the roll, dividing the pattern into dots to make a binary encoded value and then for producing a series of plot data; and PA1 (g) a control device including a detector device for use in detecting a rotational angle of the roll and instructing a pulse generating timing of the above-mentioned laser plotter on the basis of a signal sent from the detector and the plotted data.
This photo-etching process can be used to form a pattern of fine projections and indentations on a rolling roll. Particularly in processing a large roll, such as the rolling roll, however, the aforementioned process requires not only many manhours but operation in a darkroom, and moreover since it is difficult to adopt automatic operations, the process is extremely disadvantageous in manufacturing costs as compared with the aforesaid shot-blasting process or laser beam machining process.
Japanese Patent Publication No. 62-11922 discloses a process for covering a sheet surface with an acid corrosion-resistant material, locally destroying the coated surface with a laser beam, and chemically etching the locally uncovered spots. This process, however, is still too premature to be practically adopted as a substantial process.
Next, steel sheets provided with projections and indentations will be described.
In Japanese Patent Publication No. 62-11922 and Japanese Patent Laid-Open No. 62-168602 is disclosed a laser-beam dulling process wherein pulses of a laser beam are projected on the surface of a roll to form projections and indentations at a fixed pitch on the surface of the roll. According to the process, since regular projections and indentations can be formed at an arbitrary spacing on a bright roll, steel sheets that are skinpass rolled by the use of this roll are provided with level sections and projections and indented sections alternately and regularly. It is known that, in the level sections, no irregular reflection takes place; and higher sharpness of reflection is obtained rather in the skinpass-rolled sheets than in sheets rolled with the aforementioned shot-blasted roll.
The sharpness of reflection of a steel sheet surface before painting has been explained above. Next, the sharpness of reflection after painting will be described.
After chemical conversion treatment and electrodeposition painting, the sheets undergo painting by dip coating or spray washing. Generally, the painting process provides level projections and indentations on the steel sheet surface, thereby improving DOI as compared with the sheet before painting.
In vertical painting, such as the dip coating, a paint liquid applied to the sheet flows down the sheet surface with gravity. In this case, when there exist fixed paths along which the paint flows down, the paint can flow uniformly; if, however, there is any separated independent indentation, the flow of the paint will stop thereat, resulting in an uneven film thickness of coating.
As stated above, a steel sheet with projections and indentations having a large ratio of flat area in the surface thereof and having fixed paths that allow the uniform flow of the paint liquid is suitable for use because of the sharpness of reflection. A steel sheet manufactured by the laser-dulled roll satisfies at least the former, while a steel sheet manufactured by the shot-blasted roll can not satisfy both, resulting in a low DOI value.
Next the press workability will be described. Press working is done by first holding edges of a sheet between a die and a blank holder, applying a specific blank hold-down pressure to the sheet, and then punching the sheet at the center into a predetermined shape. Commonly, a material flows from the blank holding position into the die section immediately after pressing. This becomes an inflow characteristic, which is used as an index indicating the difficulty or ease of press working. For example, a material having great frictional resistance flows little from the die, resulting in a fracture thereof.
The surface of the steel sheet, when pressed, is commonly coated with a lubricating oil. It is important to obtain a substantial effect of this lubricating oil to prevent the fracture of the sheet.
In the meantime, there is a problem of galling in that a part of the steel sheet surface is galled by the die and coheres on a tool in case of a high frictional resistance when the steel material is in contact with, or slides on, the tool.
A steel sheet with a large level surface area has superior sharpness of reflection, but not necessarily satisfactory press working function.